Self-clinching nuts and pierce nuts of the type disclosed herein generally include a projecting central pilot portion, which may be used to pierce a metal panel or plate to which the fastener is attached and the pilot portion is then received through the pierced panel opening. The nut is then attached to the metal plate by a die member or "die button" which forms a mechanical interlock between the nut and the panel. The panel may be deformed by the die member into grooves in the nut or the nut may be deformed to entrap panel metal.
Many pierce nuts are used by the automotive industry to assemble cars in which many components of various kinds are attached to metal plates or panels. For example, piece nuts are used to attach lamps and sheet metal parts to the vehicle. When such parts are attached, screws or bolts are threaded into the hole or bore in the nut and a bolt or screw is tightened with a rotating tool, such as a torque wrench, at prescribed torque values. When the nut is used as a clinch nut, the nut may be attached to a preformed panel opening and the nut bore may be unthreaded to receive a thread forming or thread rolling bolt. The pierce or clinch nut must therefore have sufficient anti-torque or rotation resistance (the force that keeps the nut from rotating on the metal plate when a bolt is threaded into the nut bore and tightened) to bind the nut to the metal panel. After a component is attached to the nut on a metal plate, external forces, such a vibration and tensile forces, applied to the vehicle, act upon the nuts from the pull-through direction attempting to pull the nuts from the metal plate to which they are attached. Therefore, each pierce or clinch nut must have sufficient pull-through resistance (the force that keeps the nut from coming out of the panel when the nut and a bolt are engaged and the force is applied to the bolt perpendicular to the metal plate) that is stronger than these external surfaces.
As stated, the torque of the rotation tool or torque wrench is generally predetermined, such that the rotational resistance of the pierce or clinch nut in the panel should be sufficient to resist this torque value, but the external forces applied to the vehicle cannot be forecast. Therefore, the aforementioned pull-through resistance must be relatively high. When pierce or clinch nuts are being driven into metal plates, the nuts are supplied to the installation tooling continuously through an outlet of a supply device, such as a hopper or cartridge. Thus, it would be preferred if the shape of the pierce or clinch nuts permit free variance of the attachment orientation on the surface of the metal plate. In other words, the shape of the pierce or clinch nut should preferably permit free variation of the rotational orientation that each nut emerges from the outlet of the hopper or cartridge. In cases where pierce nuts are driven into a metal panel in a number of locations, the pierce nuts should be shaped so that the direction of the pierce nut outlet can be freely varied to suit the installation operation.
Further, in the automotive industry, for example, which utilizes many pierce nuts, there is a trend toward thinner metal panels or plates to reduce the weight of each vehicle. Thus, it is necessary to have pierce or clinch nuts shaped to provide the necessary rotation resistance and greater pull-out and pull-through resistance, even when used on thin metal plates. When, for example, it is necessary to achieve pull-out resistance in excess of 200 kg and sufficient rotation resistance to withstand the tightening torque when applied by a torque wrench with a 0.6 min. plate and the bolt or screw meets resistance during engagement of the nut, existing pierce nuts of the type described above often cannot consistently satisfy these requirements.
As described, a pierce or clinch nut is typically attached to a metal panel or plate with an installation die or die button. The die button includes one or more projecting lips configured to be received in the nut groove or grooves. When the pierce or clinch nut has an annular groove as described herein, the die button includes an annular lip configured to be received in the annular groove of the nut. When the self-attaching nut is a pierce nut, the die button typically includes a shearing edge or surface which cooperates with an outside surface of the pilot portion of the pierce nut to pierce an opening in the panel and the die button then deforms the panel into interlocking relation with the nut groove or grooves. This mechanical interlock must be sufficient to withstand the various forces described. The improved self-clinching or pierce nut described in the above-identified related applications having a protrusion in the groove bottom wall provide improved torque resistance and mechanical interlock between the nut and the panel while improving die button life.
Certain problems, however, developed in the manufacture of the embodiments of the self-attaching nut disclosed in the above-identified related applications. As will be understood by those skilled in the art, the forming punch used to form the annular groove of the self-attaching nut generally conforms to the shape of the groove bottom wall, including the protuberances described. The nut is generally formed from a metal blank, preferably a drawn metal rod of cold heading quality steel. The rod is cut to length and partially formed in a conventional cold-heading operation to form the general configuration of the nut, including the pilot portion, flange portion and an annular groove. However, when a conventional cold heading tool or punch was utilized to form the preferred configuration of the bottom of the groove, including the spaced protuberances, it was found that the tool life was insufficient for mass production applications. Thus, it was necessary to modify the method of making the self-attaching nut to improve tool life and certain modifications were also made to the nut configuration to utilize the improved method of this invention.